Opens With Assembled QFN Components

We are experiencing opens on assembled QFN components at a rate of approximately 9%. The component has two rows of pads and the failure only appears in rows near the component center. Our boards are double-side reflow and this component is mounted on the first side. What is happening?Board Talk

Transcript

Phil
And welcome to Board Talk with Jim Hall and Phil Zarrow from ITM Consulting. We are here to discuss process problems, dilemmas, situations and whatever ails you.

Jim, today we have a question from MN. I don't know if he is in Minnesota or not. MN says, we are experiencing opens on assembled QFN components at a rate of approximately 9%. The component has two rows of pads and the failure only appears in rows near the component center. He says usually only one or two pads have opens. Our boards are double-side reflow and this component is mounted on the first side. What is happening?

Obviously, there are gremlins in your reflow oven. That was easy, moving on to the next one.

I think you would agree with me Jim, it could be a multitude of situations here.

JimRight, he said assembled components at a rate of approximately 9% and then later he or she said that only one or two pads have opens. I am wondering what the 9% is. That means that 9% of all the QFN components experience this problem, as opposed to 9% of the opportunities because that would probably be more than one or two pads. 9% of the QFNs they are putting down of this particular design are having these defects.

That is still very high, that is not good. But components with two rows increases the degree of difficulty.

Phil
Absolutely. There is a lot of information we don't have here. The first question is, we don't know what he is doing on the big old thermal pad. We don't know if he is window painting. What effect he is using on that.

Jim
What percentage reduction if any. If they are using a preform. I assume you are leaning toward the possibility of floating. Where you simply have too much volume of paste on that center pad and it lifts the whole package up and some of your pads become opens.

But that raises the question of why are they in a specific location every time, the center of the component.

Phil
Then the other side of the coin, is what is his or her aperture design specs with regard to the opens on the actual joints themselves. Whether we are getting adequate deposits there.

Jim
So basically, are you getting enough paste in those areas. For some reason are you getting less paste in the area where you are seeing the opens. That would be an obvious question. Stencil would be one thing to look at. Also, the printing parameters. Board support, is there a little lack of support in the middle of that component? Is there any warpage related to the component?

Phil
Very good. That could be a major contributor there too.

Jim
Typically, QFN or BTC components are really small, under a half an inch in many cases. Warpage is not as frequent as it is in some of the large, complex BGA packages.

Phil
It is still a factor. It enters into it, it is a concern.

Jim
The question is raised when you have it in the middle. If you think of a frown, where the center of the component is higher than the corners, that might tend to increase the spacing between the board and component causing opens in the center locations.

Phil
Which brings up an interesting point, there is a movement underway to more clearly define warpage, getting away from the frown and the smile. We are actually looking at measuring warpage in all eight quadrants of a component.

Jim
Quadrants, Phil, there are four quadrants. There are eight something else, but I don't know the word for it.

There have been a number of suggested analyses put forth for more detailed quantifying of warpage, the idea that it is two-dimensional. So that people are talking about things like bowls and pipes, descriptive terms. Of course, you want to be able to quantify it. Put some numbers on it.

Phil
Think in terms rather than a smile of frown of a potato chip. That may be coming into play here. I agree with you totally with regard to the relative diminutive size of the QFN. But on the other hand, it occurs to me that we know the situation with some QFNs, It is a very thin package to begin with, being primarily plastic. And then the other thing moving away from the component solder paste in printing. What do you think about solderability of the board itself, variation there?

Jim
If you are seeing like a non-wet open where the paste stuck to the bottom of the component but didn't make good contact with the pad or the board. Or it could be the other way around where the paste soldered to the pad on the board but didn't make good contact with the pad on the bottom of the BTC component. It could be contamination on the pads of the component, either one, depending on where the open is actually formed.

Phil
And I guess last, but certainly not least would be the surface finish. Are you using HASL? HASL bad, not good.

Jim
Yeah, you are saying uneven pads across the component. That is possible but I wouldn't think it would be that consistent in one location, in the middle of the package. But who knows. HASL is a weird world of high pressure air.

Phil
The way MN states it though usually one or two pads have opens. MN hasn't really indicated it is the same two pads. So, we don't know how random these are as far as the location. It could be any of these things.

What are your thoughts, Jim, with regard to emphasis on the first side and going through the reflow excursion again? Do you think there is an effect there?

Jim
No, it either soldered the first time or it didn't. I guess if there was a marginal solderability you would get some dewetting. But that should be pretty obvious to inspect when you are digging into the problem. I think that is less likely, but possible.

Phil
Well MN, we have given you a number of things to think about. I suspect our readers will probably throw in some other ideas too.

Jim
Phil, that is such a great thing. I want to thank all of our readers for responding and sending in your comments and your expertise. We are just two guys. We haven't seen everything. We have gotten a lot of really good suggestions from our readers. Thank you very much.

Phil
So MN, we have given you a lot to think about. There is obviously more pending and we hope this goes toward resolving your QFN problem. You have been listening to Board Talk with Jim Hall and Phil Zarrow. Whatever component you are trying to solder, whatever you do, please don't solder like my brother.

Jim
Don't solder like my brother.

Comments

I am an RF board designer. Hey guys just wanted to give you positive feed back. I found this new letter about six months ago. I have seen some of the articles in the past. This is one of the few new letters a actual look forward to getting. Keep up the good work.

Chris Montoya, Humatics

Are the defects found before the second or after the first reflow? If the part test okay after the first reflow then take some masking and cover the component and allow to dry. Run the second side and recheck the connections. If the connection goes is good then you know you have an issue with the part being run upside down during the second run thru the oven!

Once you know that, then ask why? Does your paste specification tell you the maximum weight of components that can be populated on the bottom side? If it doesn't exceed the limit them you may have your bottom heat too hot or too much paste on your belly pads. Hard to figure out with the information given. Hope this helps!

Mark Maheux, Honeywell Fire Safety

There are a couple of other possibilities as the source of the problem. First, if the paste from the inner row of pin pads connects with the ground pad during reflow, the paste will pull into the larger pad. Evidence for this would be little to no solder on the open joints. Second, the presence of opens on the double-reflowed part could point to fracture or tearing during the second reflow.

Tension on pads constrained in the z-axis can be induced by nearby pads without z-axis constraints. As the temperature increases prior to reflow, CTE differences increase the tension and the solder gets weak, the intermetallic will pop out of the joint. Evidence for this is a tear at the IMC/solder boundary or evidence of reflow after separation.

Julie Silk, Keysight Technologies

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